Certain nitrophenolic compounds are sufficiently toxic to life to render them effective for use as herbicides, insecticides, or miticides. Such compounds include dinoseb (2-(1-methylpropyl)-4,6-dinitrophenol) which has been widely used as a herbicide since the 1950's on a variety of crops in the United States. Concerns for the safety of agricultural workers has resulted in discontinued use of dinoseb. However, numerous sites remain contaminated with this compound.
Other nitroaromatic compounds are similar to dinoseb in terms of chemical structure, but have other applications, such as in explosives. Such compounds include trinitrotoluene (TNT) and dinitrotoluene (DNT). Because of the widespread use of these compounds over a lengthy period of time, many sites have become contaminated with these compounds, including both manufacturing and military sites.
Many nitroaromatic compounds are either poorly degradable or nondegradable in field environments outside the laboratory. Previously, land farming was the favored method for disposing of these and other chemicals, wherein the chemicals were mixed with soil, fertilizer was added, and the mixture aerated to promote microbial activity. Unfortunately, nitroaromatics were not satisfactorily degraded by land farming or other well-aerated processes. Possible reasons include lack of nitroaromatic-degrading microorganisms, partitioning of the contaminant chemicals to biologically sequestered or inhospitable parts of the environment, and accumulation of toxic partial-breakdown by-products. Problems with land farming in general included the slow rate of biodegradation, high expense, and accumulation of toxic by-products.
Other methods have been used to remove nitroaromatics and similar compounds from contaminated soils, but with little practical success. Such methods include transportation of contaminated soil to hazardous waste dumps, and on-site incineration of the soil. Problems with such methods include high cost and poor accountability of the responsible party.
Previous laboratory studies indicated that certain nitroaromatic molecules are susceptible at least to microbiological transformation. However, the studies did not disclose biochemical mechanisms of such transformation or degradation or whether the nitroaromatic compounds were completely mineralized. In one study, for example, a soil Moraxella microorganism was isolated that was capable of growth on p-nitrophenol as its only source of carbon and energy. Spain et al., Biochem. Biophys. Res. Comm. 88:634-641 (1959). In another study, the anaerobic bacterium Veillonella alkalescens reductively transformed nitroaromatic compounds, converting the nitro groups to amino groups. McCormack et al., Appl. Environ. Microbiol. 31:949-958 (1976).
Aminoaromatic derivatives of nitroaromatics can undergo enzymatic oxidation to form polymeric (large molecular weight) materials. Parris, Residue Revs. 76:1-30 (1980). In the field, such polymers are usually incorporated into soil humic matter. Channon et al., Biochem. J. 58:70-85 (1944); McCormick et al., Appl. Environ. Microbiol. 31:949-958 (1976); Simmons et al., Environ. Sci. Technol. 23:115-121 (1989). Humic matter tends to be long-lived in soils, thereby representing a major long-term environmental fate of many nitroaromatics and aminoaromatics. Other soil microorganisms are capable of cleaving the azo linkages of polymerized aminoaromatics, often forming toxic by-products.
Bacteria are also able to attack nitrobenzoic acid, Cartwright and Cain, Biochem. J. 71:248-261 (1959), as well as o-nitrophenol and m-nitrophenol, Zeyer and Kearney, J. Agric. Food Chem. 32:238-242 (1984), where the nitro group is released as nitrite. Again, however, complete mineralization has not been demonstrated. Further, nitrite release has not been found to be a significant pathway for highly substituted nitroaromatics. No instance is currently known where a compound possessing more than one nitro substituent has been completely mineralized. In fact, the pertinent literature presents no evidence supporting ring cleavage of highly substituted nitroaromatics. Kaplan, "Biotransformation Pathways of Hazardous Energetic Organo-Nitro Compounds," in Biotechnology and Degradation. Adv. Appl. Biotechnol. Ser. 4:155-181, Gulf Pub. Co., Houston, Tex. (1990).
Aromatic groups in general appear to be degradable via only a few aerobic and anaerobic pathways. Gottschalk, Bacterial Metabolism, 2d ed., Springer Verlag, New York (1986), pp. 157-162; Berry et al., Microbiol. Rev. 51:43-59 (1987); Schink, "Principles and Limits of Anaerobic Degradation: Environmental and Technological Aspects," in Zinder (ed.), Biology of Anaerobic Microorganisms, Wiley, New York (1988). Aerobically, many aromatic groups are degraded to catechol, protocatechuate or homogentisate by the action of oxygenase and dioxygenase enzymes. Catechol and protocatechuate can be degraded further by aromatic ring cleavage either ortho or meta to the hydroxyl groups. Because of the difficulty of working with anaerobic microorganisms and processes, biochemical pathways describing anaerobic degradation of aromatic compounds have been less well characterized.
Alkyl groups on aromatic rings are degradable via reactions similar to those for simple alkanes. Under aerobic conditions, the terminal carbon is oxidized to yield a carboxylic acid. Degradation then proceeds by .beta.-cleavage to yield either benzoates (odd-numbered carbon chains) or phenylacetates (even-numbered carbon chains). No anaerobic microorganisms capable of carrying out this process have been isolated to date. In spite of the above results known in the art, there is little information currently available on practical means of using microbial cultures to bioremediate nitroaromatic-contaminated soils.
Dinoseb, an intensely yellow-colored compound visible at concentrations as low as 10 ppm, has been found to not significantly accumulate in agricultural soil at normal application rates, even after years of repeated application. Doyle et al., J. Agric. Food Chem. 26:987-989 (1978). However, higher application rates, such as from spills of substantial amounts of the compound, can result in appreciable accumulation at a site. Presumably, therefore, dinoseb at lower concentrations is transformed by certain soil microorganisms. Such transformation appears to result only in the formation of amino and acetoamido forms of dinoseb, which apparently retain significant toxicity. Parris, Residue Revs. 76:1-30 (1980).
Previous work on the biotransformation of the explosive 2,4,6-trinitrotoluene (TNT) indicates that the primary mode involves transformation (reduction) of the nitro groups. Kaplan, supra. A recent paper from Soviet researchers describes degradation of TNT by a strain of Pseudomonas fluorescens. Naumova et al., Mikrobiologiya 57:218 (1988). But, while these reports shed some light on microbial events and hypothetical biochemical mechanisms therefor, they neither disclose nor suggest effective methods for bioremediating soils or wastewater contaminated with these compounds. Further, the Soviet results have not been confirmed outside the U.S.S.R.
Hence, although several anaerobic microbiological systems have been described for degrading other aromatic chemicals, little to no information is available on practical means of using these cultures to bioremediate contaminated soils and waters, especially soils and waters contaminated with nitroaromatics. In today's world, effective remediation of environmental sites contaminated with compounds, such as nitroaromatics, requires that the contaminants be completely mineralized to ensure the absence of latently toxic by-products. Such results for nitroaromatics simply have not been shown in the prior art, particularly as applicable to large-scale, low-cost bioremediation efforts.
Therefore, there remains a need for a method to effectively bioremediate dinoseb-contaminated soils, as well as soils contaminated with other nitroaromatic compounds, such as TNT and DNT.
Further, there is a need for such a method that can be performed at a natural site contaminated with dinoseb or a related nitroaromatic compound.
Further, there is a need for such a method that can completely degrade dinoseb and other nitroaromatics, leaving no detectable or environmentally significant amounts of aromatic by-products or other toxic intermediary compounds, including polymeric derivatives.
Further, there is a need for such a method employing microorganisms of types and species profiles normally found in many soil environments.
Further, there is a need for such a method that is inexpensive and easy to perform, particularly on a large-scale, in the field.
Further, there is a need for such a method that can be performed rapidly, including in the field.
Further, there is a need for such a method that will effect bioremediation of nitroaromatic-contaminated soil without specialized bioreactors or other complex equipment.